Uprights and transverse braces are principal components of a knock-down scaffold frame. A variety of releasable joints are known that permit the principal components to be joined to produce a unitary structure. The present invention has application to a wedge-operated joint whose general configuration has been widely accepted in the scaffold industry. The joint includes an annular horizontal flange which is fixed to an upright and which has a set of openings between its upper and lower faces. A complementary connector is fixed to the brace and has a mouth that receives the flange. A vertical passage in the connector is aligned with a flange opening, and a wedge is inserted through the aligned passage and flange opening to secure the joint. The wedge acts between the connector and the radial periphery of the flange to force the connector against the upright, locking the joint. Exemplary joints of this nature are shown in U.S. Pat. No. 4,044,523, No. 4,394,095, No. 4,426,171 and No. 4,603,756, all to Layher.
Inserting the wedge causes wear in the flange, the connector and the wedge itself. The wedge normally bears against a particular seating surface formed in the flange opening. In the prior Layher patents, the seating surface is rounded and the edge of the wedge that engages the seating surface is inclined relative to vertical. Since the wedge is driven vertically, it attacks the point of contact with considerable force, producing essentially a cutting action. Although the wedge is shaped to mate with vertical surfaces within the connector, at least when fully seated, similar point contacts may be formed as the wedge is driven to a final position or during initial stages of removal while the joint is stressed. The resulting wear may be gradual and tolerable in steel scaffold frames but it can quickly defeat the operation of an aluminum system. In that regard, it should be noted that steel wedges are still suggested for use in aluminum systems in order to withstand the hammering necessary to insert and remove the wedges.
Such scaffold joints also have a tendency to loosen and release. Wedging action inherently applies an upward force to the wedge. Point contacts between components may be inadequate to react such upward forces and avoid loosening of the wedge. Another significant aspect of this problem appears to have been overlooked. The connector normally rests on the upper surface of the flange when the joint is in a released state. As the wedge is driven downwardly, it not only forces the connector horizontally against the upright but also raises the connector until the latter engages the lower surface of the flange. Vibration, striking or other dynamic loads applied to the associated brace then cause the connector to dislodge, releasing the wedge and the joint. Moreover, a worker senses a significant resistance to wedge insertion when the connector engages the bottom surface of the flange. A worker tends to strike the wedge several more times assuming, incorrectly, that the wedge is just beginning to lock the joint. This encourages further damage to the components of the joint and is most significant if an aluminum flange is provided.
Alternative wedge-operated scaffold joints have been proposed but these also appear subject to the wear and loosening problems described above. U.S. Pat. No. 4,493,578 to D'Alessio proposes a wedge with one rounded side edge ostensibly conforming to vertically-aligned surfaces in the connector. The opposing side edge of the wedge and the corresponding seating surface within the associated flange opening are both angled relative to vertical, somewhat reducing wear. However, the angle between the side edges of the wedge does not conform to the inclination of the wedge-seating surface defined by the flange. Point contacts are formed, which contribute once again to wear. Furthermore, producing a wedge-seating surface that is not perpendicular to the customary flat upper surface of the flange adds complexity and increases manufacturing costs. In U.S. Pat. No. 4,840,513 to Hacket, point contacts are formed between the wedge, connector, and flange at various stages of wedge insertion and removal, once again leading to wear. In the scaffold joint described in U.S. Pat. No. 4,525,096 to Green et al, the wedge has one edge that aligns with a corresponding vertical wedge-seating surface within a flange opening, once the wedge is fully seated. However, the opposing side edge wedge is then angled relative to vertical and bears against vertical surfaces in the connector. One of the vertical connector surfaces is formed on an abutment element that displaces relative to the rest of the connector. Once again, point contacts are formed during wedge insertion and removal, which are expected to lead to wear.